This invention relates to electrical fuses, and more particularly, to high voltage current limiting fuses that provide protection for an electrical transformer subjected to short-circuit, low overload and high overload current conditions.
It is desirable that high voltage current limiting fuses used to protect electrical devices, such as transformers, be adapted to the current flowing within the environment of the transformer. High voltage current limiting fuses for electrical transformers may typically be subjected to and expected not to melt or rupture during the occurrence of a surge or inrush current corresponding to 25 times the transformer rating for a relatively short time duration of 0.01 seconds. Similarly, the current limiting fuse may be expected not to melt or rupture during the occurrence of an inrush current corresponding to 12 times the transformer rating for a relatively long time duration of b 0.1 seconds. However, under short-circuit conditions the high voltage fuse is desired to rupture so as to prevent damage to the electrical transformer. Furthermore, it is desirable that under short-circuit conditions the current limiting fuses rupture quickly so as to reduce or limit the amount of energy "let-through" the fuse that may damage the transformer.
Still further, it is desired that a fuse be capable of clearing all fault currents from a maximum interrupting rating down to those which cause fuse melting in one hour or more. A further requirement of fuses designed to protect transformers is the ability to melt relatively quickly when subject to a fault current corresponding to a short-circuit on the output of the transformer. Since this current may correspond to only 8 times rated current and require clearing in less than 2 seconds, it can be seen that many of these requirements impose conflicting demands on the fuse designer.
High voltage current limiting fuses are well known. One such high voltage current limiting fuse is described in U.S. Pat. No. 4,198,615 issued to W. R. Mahieu on Apr. 15, 1980. The fuse of the Mahieu patent has a plurality of current limiting elements and a plurality of arc gap establishing means both electrically coupled in parallel. Upon the occurrence of low current fault conditions the current limiting fuses sequentially distribute the fault current to the parallel arranged fuse elements one at a time to cause relatively fast melting of each of the fuse elements so as to enhance the clearance of low fault current conditions. It is considered desirable to accomplish the function of proper current limiting by the use of fuse elements alone and to reduce the number of required fuse elements.
A high voltage fuse comprising a plurality of similar fuse elements connected in parallel is described in U.S. Pat. No. 3,835,431 entitled "Electrical Fuse", and issued to Philip Rosen et al, Sept. 10, 1974. The Rosen et al electrical fuse provides protection for short-circuit, low overload and prolonged low overload current conditions.
A still further current limiting fuse is described in U.S. Pat. No. 2,866,037 entitled "ELECTRIC CURRENT LIMITING FUSE", issued to V. N. Stewart, Dec. 23, 1958. The Stewart current limiting fuse has constricted portions of reduced cross-sectional area for reducing arc energy and also an alloy-forming material for improving the response of the fuse to the occurrence of low, protracted overload current conditions. Neither Rosen et al or Stewart is adapted to discriminate between fault and transient or surge conditions. It is considered desirable to provide a fuse which is adapted to discriminate between a fault and a surge or transient and abnormal rush of current conditions into an electrical device. Under fault condition the fuse ruptures whereas under surge conditions the fuse withstands the surge and does not rupture.
Accordingly, it is an object of the present invention to provide a high voltage current limiting fuse that provides proper protection of an electrical device such as a transformer during short-circuit current conditions and high or low overload current conditions.
It is another object of this invention that the fuse withstand a wide range of current surges without rupturing.
It is a further object of this invention that the fuse elements within the fuse rupture quickly under short-circuit conditions so as to reduce the amount of energy "let-through" by the fuse.
These and other objects of this invention will become apparent to those skilled in the art upon consideration of the following description of the invention.